This invention relates to converting a stream of signal samples appearing at one sample rate into a stream of signal samples that carry the same information but appear at a different sample rate.
In modems used for full duplex communication over a two-wire voice-band telephone line, for example, the process of echo cancellation requires sampling the line signal at a first sampling rate that is related to (typically a multiple of) the local transmitter's baud rate. The sample stream is then subjected to echo cancellation by subtracting a sample stream (appearing at the same first sampling rate) that represents echo and leakage versions of the locally transmitted signal.
For decoding, however, the echo cancelled sample stream must be converted to a new sample stream that carries the same information but appears at a second sampling rate that is related to (typically a multiple of) the remote transmitter's baud rate. Commonly the remote and local transmitter baud rates are nominally the same but actually differ slightly.
It is known to accomplish sample rate conversion in the analog domain by converting the echo cancellation signal to analog, subtracting it from the analog received line signal, then reconverting the result to the digital domain at the desired second sampling rate, for decoding.
Alternatively, the echo cancellation may proceed in the digital domain (at the first sampling rate). The digital result is then converted to analog and reconverted to digital (at the second sampling rate).
It is well known that if an analog signal is sampled at a sufficiently high rate, the resulting samples will contain enough information to permit the original signal to be reconstructed perfectly. It is also known that by passing the samples through a transversal filter having a properly chosen set of coefficients, a new sample stream can be generated such that the new sample stream appears at any desired sample rate and, like the original sample stream, accurately represents the information in the original analog signal.
Miller, U.S. Pat. No. 4,453,259, shows a modem in which samples of the line signal appearing at a first fixed rate are passed through an interpolation filter to generate a revised sample stream at a second rate. By selecting an appropriate one of several sets of filter coefficients, the second sample rate can be made to be any one of several different rates.
Weinstein, U.S. Pat. No. 4,131,767, shows an echo cancellation digital modem for full duplex communication in which the line signal is sampled at the Nyquist rate, then echo cancelled, and then resampled at the baud rate. One technique suggested for accomplishing the resampling is an "interpolation formula".